In recent years, there is an increasing need of downsizing mobile machines and of effective usage of electrical energy generated from natural resources. Therefore, research and development is underway to achieve lithium ion secondary batteries (as a type of electrical storage devices) that can provide higher voltage and have high energy density.
As packaging materials for such lithium ion secondary batteries as described above, metal cans have typically been used. However, metal cans are being replaced by pouched laminates, as packaging materials, which are composed of a lamination of a metal layer (e.g., aluminum foil) and a resin film. This is because such pouched laminates can cope with the needs for thinner and diversified products to which the lithium ion secondary batteries are applied, and can be manufactured at low cost.
A lithium ion secondary battery includes a battery body, a packaging material wrapping the battery body, metal terminals (tab leads) each connected to a negative or positive electrode of the battery body and extended to the outside of the packaging, and a terminal film (may often be called tab sealant) for the electrical storage device, partially covering an outer peripheral surface of each of the metal terminals.
A part of the electrical storage device terminal film is covered with a packaging material while the rest of the film is exposed from the packaging material. The electrical storage device terminal film is subjected to a thermal adhesion process for adhesion to the metal terminal.
The electrical storage device terminal film is used for insulating a metal layer composing the packaging material from the metal terminals. Therefore, the electrical storage device terminal film is desired not only to achieve adhesion between the terminal film and the packaging material and between the terminal film and the metal terminals, but also to have sufficient insulating properties after the heat sealing process (in other words, to have a thickness that can ensure sufficient insulating properties).
If the electrical storage device terminal film is transparent and the interposition of the terminal film between the packaging material and each of the metal terminals is going to be checked, it is difficult to determine the presence of the terminal film.
Further, if the electrical storage device terminal film is transparent and the position of the terminal film relative to the metal terminals is going to be checked, it is difficult to accurately determine the position of the terminal film.
As a structure for such an electrical storage device terminal film, a single layer structure or a lamination structure (e.g., see PTL 1) is known.
PTL 1 discloses an electrical storage device terminal film (film for lead wires) including a lamination of three layers each made of polypropylene graft-modified by unsaturated carboxylic acid, or polyethylene graft-modified by unsaturated carboxylic acid.
Known methods of fabricating the above-mentioned terminal film for electrical storage device include, for example, extrusion, such as inflation molding, using a round die, or die pressing using a T-die, and the like. The above-mentioned terminal film for electrical storage device is fabricated using a film-extrusion fabrication apparatus having a die, such as a round die or a T-die.
An electrical storage device terminal film fabricated by means of such an apparatus (specifically, a film-extrusion fabrication apparatus or the like) is taken up by a take-up roller configuring the apparatus, conveyed and stored.
When using (processing) the electrical storage device terminal film taken up by the roller, one end of the terminal film in a roll is drawn out.
PTL 1 also discloses that the electrical storage device terminal film (film for lead wires) is colored using a pigment or a dye to accurately check whether the terminal film is interposed between the packaging material and the metal terminals.
PTL 1 also discloses that pigments for coloring the electrical storage device terminal film include extender pigments as inorganic pigments, such as calcium carbonate, silicic anhydride, alumina and cobalt blue, white pigments, such as titanium dioxide and zinc oxide, and black pigments, such as iron black and black lead. PTL 1 further discloses use of insoluble azo pigments as organic pigments, or azo pigments such as azo lake pigments, phthalocyanine pigments such as copper phthalocyanines and metal-free phthalocyanine lakes, anthraquinone pigments, condensed polycyclic pigments such as thioindigo pigments, as well as nitroso pigments, daylight fluorescent pigments, and the like.